Sulfur deposits occur in nature in two forms: (1) large, deep continuous beds of nearly pure sulfur, often in the form of inverted cones; and (2) surface deposits consisting of solid mixtures of elemental sulfur and gangue.
Sulfur found in large deep beds has traditionally been mined by a method known as the "Frasch process" which involves forcing superheated water down drill holes into the sulfur bed to melt the sulfur and pumping molten sulfur to the surface by compressed air. The sulfur deposit must have a capping, usually of rock. strong enough to hold the superheated water down into the bed under high pressure. Although the sulfur thus produced is very pure, the recovery of the total sulfur by this method is usually in the neighborhood of 30 to 40 percent. The inefficiencies of the Frasch method were overlooked in the past because of the then abundance of suitable sulfur deposits. These sources of sulfur have, however, dwindled to a point where today we must look to alternate sources of sulfur (e.g., surface deposits) as well as explore new methods to maximize recovery of sulfur from large deep deposits.
Throughout the years, many techniques for separation of sulfur from surface ores have been devised. The usefulness of these techniques depend largely upon the form and amount of sulfur present in the ore, as well as the efficiencies of the technique and purity of the resultant sulfur. One method, known as the "Sicilian process", produces very pure sulfur and involves actually burning the ore. The heat melts the sulfur which is collected in molten form. The combustion is fueled by the sulfur itself, thus a quantity of the sulfur contained in the ore is consumed as fuel. Sulfur recovery is further limited by the tendency of molten sulfur to adhere to gangue. In practice, application of the Sicilian method has been limited to those types of ores containing substantial veins of pure sulfur. Adherence of molten sulfur to gangue renders the process unsuitable for ore having small sulfur deposits disseminated throughout, the form in which the majority of sulfur is found.
Other methods used on surface ores include the vaporization, solvent extraction methods, flotation, autoclaving, agglomeration and various combinations of the same. The vaporation process involves heating a slurry of ground ore to vaporize the sulfur contained therein. The sulfur and water constituents of the slurry are separated from the solid gangue constituents as vapor. However, the process is quite expensive to carry out due to the amount of energy required to vaporize sulfur.
The solvent extraction process involves removing sulfur from surface ores with the use of solvents in which sulfur is partially soluble. Solvents which have been used include carbon disulfide, kerosene and chlorinated hydrocarbons such as trichlorethylene and perchlorethylene. Ammonia has also been suggested as a suitable solvent for the process.
In the flotation method, ore is finely ground and mixed with water. Sulfur particles are separated from gangue particles based upon solid sulfur's ability to float in solution while gangue sinks to the bottom. Modifications of this method include adding a reagent to increase sulfur's floatability in water. Reagents which have been used include creosote, pine oil, kerosene, alcohols and methyl isobutyl carbinol. This process, however, produces sulfur of very low purity, especially where the sulfur in the starting material ore is extremely disseminated therein.
The autoclaving method is somewhat like an artificial Frasch process in that it involves the use of superheated water to melt sulfur off of gangue. Fines of ore are placed upon a grate and superheated water and live steam are added to melt sulfur off of gangue. The molten sulfur falls through the grate and is collected below. Modifications of this method include using saturated aqueous solutions of calcium chloride or zinc chloride as the autoclaving medium. These solutions have densities above that of molten sulfur and boiling points above that of sulfur. The sulfur coalesces in a pool above the medium while the gangue is attracted to the medium by gravity and surface tension.
One such method of sulfur extraction is that of U. S. Pat. No. 2,731,332 to G. F. Ackert et al. which discloses a method to extract sulfur from ore in the molten state. In that method, a slurry comprising only sulfur ore and water is heated to 280 to 300 degrees Fahrenheit under pressure to cause the sulfur constituent of the ore to melt. The water component of this slurry functions only to form the slurry and as a heat transfer medium to allow the sulfur constituent to melt. The heated slurry, which consists at this stage of water, gangue and molten sulfur, is then passed through a filter comprised of a metallic screen and a layer of wet gravel. The filtrate of this first separation consisting of sulfur, water and sludge is collected below the screen and led to a settling tank. The filtrate then undergoes a second separation whereby it is allowed to settle into two phases: (1) a water phase; and (2) a molten sulfur and sludge phase. The sludge particles of the second phase, due to their densities, collect at the bottom of the sulfur reservoir and are later drawn off. The water component of the filtrate is recycled into the process after it has been purified by a flocculation process to remove any suspended solids contained therein by colloidal action. In the water purification step, certain unspecified "chemicals" are introduced into the water filtrate to control pH and/or flocculation of the suspended solids.
The acid agglomeration process has been used to purify sulfur, especially that found in flotation concentrates. The process essentially involves agglomerating gangue particles with strong (98%) sulfuric acid. The flotation concentrate is first ground and melted at atmospheric pressure. Sulfuric acid is then added and the mixture is slowly agitated to cause the molten sulfur to coalesce on the bottom and the gangue particles to agglomerate on the top. This process, however, was considered unattractive because of the dangers involved in handling large amounts of hot sulfuric acid.
The observation that molten sulfur droplets tend to coalesce in both the autoclaving and acid agglomeration methods was useful in leading to the development of a process in which sulfur is melted away from gangue in an aqueous slurry heated under pressure while agitated. Forbath, T.P., "Sulfur Recovery From Low-Grade Sulfur Deposits," Transactions AIME (September 1953), pp. 881-885. After the heating period, cold water is injected into the agitated mixture and the temperature is reduced to freeze all the liquid sulfur particles. The solid sulfur is then further processed according to the flotation method. It was found that the thermal treatment step obviated the need to finely grind the ore as required by straight flotation alone which resulted in significant conservation of energy used to grind the ore and hence reduced costs of operation.
Despite the many and varied processes for separation and purification of sulfur that are known, there is still a need for a more effective process for separating sulfur from low grade surface ore. The present invention addresses this need.
It is thus an object of the present invention to provide an improved process of separating substantially gangue-free sulfur from low grade ore.
It is another object of the present invention to provide a reagent to facilitate coalescence of molten sulfur droplets in an aqueous medium.
It is yet another object of the present invention to provide a reagent which allows molten sulfur to be separated as a single continuous layer from an aqueous slurry.